1. Field of the Invention
The present invention relates to hydraulic systems for operating machinery that have a plurality of functions, each having a separate hydraulic actuator; and more particularly to such systems that operate in a regeneration mode in which pressurized fluid exhausted from one function is routed to power another function.
2. Description of the Related Art
A wide variety of machines have a plurality of moveable members operated by separate hydraulic actuators, such as a cylinder and piston arrangement, controlled by a valve assembly. Conventionally, the valve assembly controls the flow of pressurized fluid into one chamber of the cylinder and the flow of fluid from the other cylinder chamber. Which cylinder chamber receives the pressurized fluid determines the direction of motion of the machine member. The velocity of the piston, and thus the machine member, can be varied by proportionally controlling at least one of those flows.
For that proportional fluid control, the hydraulic actuator is part of a hydraulic circuit branch that has a pair of proportional electrohydraulic valves coupling each cylinder chamber to a supply conduit and another pair of similar valves connecting the cylinder chambers to the tank return conduit. The valves are operated independently, such as by the velocity based method described in U.S. Pat. No. 6,775,974 for example. In that method, the machine operator designates a desired velocity for the hydraulic actuator by manipulating an input device which sends an electrical signal to a system controller. The system controller also receives a sensor signal indicating the amount of force acting on the hydraulic actuator. The desired velocity and force signals are used to determine an equivalent flow coefficient which characterizes fluid flow in the hydraulic circuit branch. From the equivalent flow coefficient, first and second valve flow coefficients are derived and then employed to activate the two of the proportional electrohydraulic valves which control fluid flow to produce the desired motion of the hydraulic actuator. The flow coefficients characterize either conductance or restrictance in the respective section of the hydraulic system. The valve flow coefficients are converted into electrical currents that open the respective valves to produce the associated flow level.
During powered extension and retraction modes of operating the hydraulic cylinder, fluid from a supply conduit is applied to one cylinder chamber and all the fluid exhausting from the other cylinder chamber flows into a return conduit that leads to the system tank. Under some conditions, an external load or other force acting on the machine enables extension or retraction of the cylinder/piston arrangement without significant pressure from the supply conduit. In a backhoe for example, when the bucket is filled with heavy material, the boom can be lowered by the force of gravity. That force drives fluid out of one chamber of the boom cylinder through the valve assembly and into the tank return conduit. At the same time, an amount of fluid is drawn from the supply conduit through the valve assembly into the other cylinder chamber which is expanding. However, the supply conduit fluid does not have to be maintained at a significant pressure in order for that latter fluid flow to occur. In this situation, the fluid is exhausted from the cylinder under relatively high pressure, thereby containing energy that normally is lost when the pressure is released in the tank.
To optimize efficiency and economical operation of the machine, it is desirable to use the energy of that exhausting fluid, instead of releasing it unused into the tank. Under the proper pressure conditions in some hydraulic systems, fluid being exhausted from one cylinder chamber is routed by the valve assembly to the other cylinder chamber that is expanding. This mode, referred to as “self regeneration”, employs the energy of the exhausting fluid to at least partially fill the expanding chamber thereby reducing or eliminating the quantity of fluid from the supply conduit.
Continuing the example of a backhoe, as the boom is lowering, the machine operator may be raising the backhoe arm which requires that fluid under pressure be applied to the hydraulic cylinder for the arm. Therefore, the arm actuator is consuming energy, while the boom cylinder is releasing energy. It would be advantageous if the energy of the exhausted fluid could be channeled to the arm cylinder either to power that cylinder entirely or at least to augment the pressurized fluid furnished by the pump, an operation commonly referred to as “cross function regeneration.” In this case the energy from one function may be more efficiently used by another function, than used by the same function in the self regeneration mode. U.S. Pat. No. 6,502,393 describes a hydraulic system that can operate in several modes, including the cross function regeneration mode.
All the various operating modes may not be viable at a given point in time depending on the pressure conditions existing in different sections of the hydraulic system and the external forces acting on components of the machine. Therefore, it is desirable to provide a mechanism that determines which operating modes are currently viable and automatically selects the most economical one that is available.